Shutterstock For the latest study, researchers at University of Texas Southwestern Medical Center looked at a sample of older people who showed early signs of memory loss and were at risk of developing Alzheimer's. The less frequently the participants exercised, the weaker the connections in their brain's white matter and the more poorly they performed on a bunch of cognitive tests.

Exercise may help keep the brain young

As we age, the brain — like any other organ — begins to work less efficiently, so normal signs of decline begin to surface. Our memory might not be quite as sharp as it once was, for example.

Exercising regularly as we get older appears to help defend against some of this decline, both for healthy people who show normal signs of aging and for older people who may be on the path toward developing Alzheimer's disease.

Researchers still aren't sure why this is, or how it happens. Exercise could strengthen some of the pathways our brain uses to relay signals for recent events, or boost the size of certain brain regions that are key for learning and storing memories.

Most studies focusing on people with MCI require people to either work out or self-report their own fitness levels. But the latest study measured how fit people were by studying their breathing and heart rate. The researchers then used brain imaging to measure the functionality of peoples' white matter and had them take a series of cognitive tests designed to measure how sharp they were.

Overall, they found that the less fit people were, the weaker their brain's white matter connections, and the worse they did on the cognitive tests.

Two other recent studies of older people with MCI have suggested that merely amping up one's workout routine with the right moves could help slow the brain's decay.

Last May, scientists recruited adults with MCI between the ages of 60-88 and had them walk for 30 minutes four days a week for 12 weeks. The results showed strengthened connectivity in a region of the brain where weakened connections have been linked with memory loss. That development, the researchers noted, "may possibly increase cognitive reserve," but more studies are needed.

Large groups of researchers are taking note of these promising findings. In December, the American Academy of Neurology updated its guidelines to reflect the takeaways of these findings. Based on a series of 6-month studies on aerobic workouts and memory in people with MCI, the new guidelines recommend that people diagnosed with the condition do some form of cardio exercise at least twice a week.

The reason aerobic workouts lift our spirits seems related to their ability to reduce levels of natural stress hormones, such as adrenaline and cortisol, according to a study in the Journal of Physical Therapy Science. Activities like running and swimming also increase overall blood flow and provide our minds fresh energy and oxygen — another factor that could help us feel better.

Flickr / Dave RosenblumAerobic exercise may also have a uniquely powerful positive impact on people with depression. A pilot study in people with severe depression found that just 30 minutes of treadmill walking for 10 consecutive days was "sufficient to produce a clinically relevant and statistically significant reduction in depression."

So whether you're looking for benefits related to mood or memory, the take-home message is clear: the more you move, the healthier you may be.

"It's exciting that exercise may help improve memory at this stage, as it's something most people can do and of course it has overall health benefits," Ronald C. Petersen a fellow of the American Academy of Neurology and the lead author on the most recent guidelines, said in a statement.

While some benefits of exercise can emerge just a few minutes into a sweaty workout, others might take several weeks to crop up. That means that the best type of fitness is any aerobic exercise that you can do regularly and consistently for at least 45 minutes at a time.